No, you can not do this to all of them "at once". You WILL have to open each one separately. There is no other way to action each document.

That being said, you CAN use VBA to action each document unattended. Use the DIR function. Here is how. Note all the documents must be in the given folder the DIR is pointing to. - in the example below this is C:Yadda\Blah.

First write, or record, the code to insert the watermark. Name it something like AddWatermark.

Then use something like this.

Code:

Sub BunchOfWatermarks()
Dim MyFile
Dim myPath As String
myPath = "C:Yadda\Blah\"
MyFile = Dir(myPath & "*.doc")
Do While MyFile <> ""
   Documents.Open MyFile
   Call AddWatermark
   ActiveDocument.Save
   ActiveDocument.Close
   MyFile = Dir()
Loop
End Sub

All files in the C:Yadda\Blah folder will be opened, the code for AddWatermark executed (thus inserting the watermak), saved and closed, and then the next file processed.